1. Field of the Invention
The present invention relates to the casting of high temperature metals and alloys and, more particularly, to molds for use in such processes.
2. Description of the Prior Art
The use of preformed, multi-part molds for casting high temperature metals and alloys, such as nickel base superalloys, is shown in U.S. Pat. Nos. 3,888,301; 3,927,710 and 3,965,963 all of which are assigned to the assignee of the present invention. These molds may be two part molds, including preformed opposed outer mold elements which define a mold cavity therebetween, or three or more part molds, including preformed outer elements with a central mold element or strongback therebetween, the elements defining mold cavities on opposite sides of the central element. The two part mold is generally used to make a unitary article whereas the three part mold is used to cast mating article shapes which are subsequently bonded together to form a finished product, an example of the latter being cast blade or vane halves which are bonded together to form a complete hollow turbine blade or vane. Regardless of the number of parts, the outer elements of these molds have projecting mating flanges on opposite edges by which flanges the outer elements may be clamped or otherwise held together in working relationship for casting.
Although the preformed, multi-part molds may be utilized in virtually any casting process, they are especially useful in casting columnar grained and single crystal metallic articles by the processes disclosed in VerSnyder U.S. Pat. No. 3,260,505 and Piearcey U.S. Pat. No. 3,494,709. Typically in such processes, molten metal is introduced into the preformed, multi-part mold via a pour cup either permanently or temporarily affixed to the top portion of the mold above the mold cavity or cavities, the molten metal being allowed to flow or cascade downwardly in the mold against a chill plate on which the mold rests. However, introduction of metal in this manner is disadvantageous in several ways. First, such top feeding or pouring of the molten metal directly into the mold causes severe turbulence and can lead to rather severe erosion of the mold walls and strongback. Such erosion not only may adversely affect the dimensional features of the mold elements but also may introduce numerous ceramic particulate inclusions into the molten metal and eventually into the solidified casting where the inclusions are harmful to the mechanical and other properties of the casting. Second, no means are provided for flushing or otherwise removing such particulate inclusions or those which might be present originally in the mold in the form of dust, dirt and the like. Third, top feeding of the molten charge into the mold may cause breakage of the preformed strongback as a result of the sudden force of molten metal impingement thereagainst. Fourth, there is little opportunity for controlling the metal flow characteristics, including flow rate, as the mold is being filled, thereby making attainment of the desired grain nucleation and orientation more difficult. In general, these and other problems associated with the top feeding of molten metal cause reduced yields and increase the cost of casting.